The present invention concerns a loudspeaker diaphragm and a method for making such a diaphragm. More particularly, but not exclusively, this invention concerns a loudspeaker diaphragm comprising a woven fibre body supporting a damping material. The invention also concerns a loudspeaker drive unit and a loudspeaker enclosure.
GB 1 491 080 (by B&W Loudspeakers Limited—or “B & W”) discloses a loudspeaker diaphragm made from an open mesh woven fibre material, for example Kevlar®, so stiffened with a thermosetting resin that spaces are left between adjacent fibres. The spaces are partially filled with a damping material, such as PVA (polyvinyl-acetate) emulsion. The spaces between the threads of the fabric enable good bonding between the PVA emulsion and the woven fibre material. The UK company, Bowers & Wilkins have commercialised a mid-range drive unit incorporating a loudspeaker diaphragm made from a woven Kevlar® fabric, stiffened with resin, and coated with PVA. The PVA material is brushed onto the woven fibre material in one or more layers, typically resulting in the PVA material forming about 10% to 15% of the total mass of the loudspeaker diaphragm. The result is a semi-flexible cone (hereinafter, “B&W′s Kevlar cone”), which exhibits useful break-up behaviour, less coloration, and more even dispersion of the sound emitted, as will now be explained in further detail.
Continued vibration of a loudspeaker diaphragm, independent of the applied input signal, can lead to “time-smearing”—a form of coloration—and resultant impairment of the clarity of the sound produced in response to a given input signal, and of the accurate reproduction of the sound from the input signal. The PVA material provides damping, but the non-isotropic properties of B&W's Kevlar cone are cited as important: being woven, the mechanical properties of B&W's Kevlar cone are different depending on the angle to the direction of the fibres. Sound waves travel through the material of the cone at different spends depending on the direction of travel. As such, reflections of sound waves travelling across the body of B&W's Kevlar cone, happen at different times around the edge of the cone, leading to a less symmetrical pattern of sound waves, and reduced impact on sound from formation of standing waves. Less sound is received by the listener than would otherwise result from delayed energy being radiated by the cone. As a result, there is less of the undesirable “time-smearing” noise. The cone thus produces emitted sounds which are significantly clearer and which can deliver finer detail. Design details stated as providing control over the quality of sound reproduction include the type of weave, the cone geometry, and the choice of type of stiffening resins and damping materials.
B&W's Kevlar cone is used in many of B&W's products, it being widely used in the mid-range drive units supplied in B&W's loudspeakers. Kevlar has not only the above-mentioned beneficial properties but conveniently has an attractive and distinctive appearance, which makes it suitable for use as the forward-facing sound-emitting surface of the diaphragm of a loudspeaker drive-unit. It is however an expensive material and it would be useful to have an alternative material for use that could be employed in a manner that provides similar or better acoustic performance. It would also be beneficial for such a material, not only to fulfil the technical performance and satisfy the technical characteristics required of it, but also to have an outward appearance that is suitable for use within a hi-fi context.
The present invention seeks to mitigate one or more of the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved loudspeaker diaphragm. Alternatively or additionally, the present invention seeks to provide an alternative to the B&W's Kevlar cone as described above, with substantially the same or better acoustic performance.